Driving method of liquid crystal display device

ABSTRACT

It is an object to provide a liquid crystal display device and a driving method of a liquid crystal display device in each of which deterioration of an image display function can be suppressed and power consumption can be sufficiently reduced. In the liquid crystal display device, a fixed potential is input to a capacitor before a power source is turned off, so that a potential difference between electrodes of the capacitor disappears (capacitance becomes almost zero) such that electric field is not applied to liquid crystals, whereby the liquid crystals are in an initial state. When the supply of the power source is stopped after an initial-state image is displayed, unnecessary electric field is not continuously applied to the liquid crystals in an off state, whereby the liquid crystals can be in the stable initial state; therefore, the liquid crystals can be prevented from deteriorating.

TECHNICAL FIELD

The present invention relates to a method for driving a liquid crystaldisplay device and a liquid crystal display device.

BACKGROUND ART

A technique for forming a thin film transistor (TFT) by using asemiconductor thin film formed over a substrate having an insulatingsurface has attracted attention. Thin film transistors are applied to awide range of electronic devices such as integrated circuits (IC) orimage display devices (display devices).

As examples of electronic devices using thin film transistors, mobiledevices such as a mobile phone or a laptop computer can be given. Forsuch a mobile electronic device, power consumption that affectscontinuous operation time is a big problem. Also for a television setwhich is increasing in size, it is important to suppress the increase inpower consumption associated with the increase in size.

In a display device, when image data which is input to a pixel isrewritten, operation of writing the same image data is performed onceagain even in the case where image data in a period is the same as thatin the preceding period. As a result, by performing the operation ofwriting the same image data a plurality of times, power consumption isincreased. In order to suppress such increase in power consumption in adisplay device, for example, a technique has been disclosed in which anidle period which is longer than a scanning period is set as anon-scanning period every time after image data is written by scanning ascreen in the case of displaying a still image (for example, see PatentDocument 1 and Non-Patent Document 1).

REFERENCE Patent Document

-   [Patent Document 1] U.S. Pat. No. 7,321,353

Non-Patent Document

-   [Non-Patent Document 1] K. Tsuda et al., IDW '02, Proc., pp. 295-298

DISCLOSURE OF INVENTION

However, with the display method in which a still image display ismaintained by setting an idle period which is longer than a scanningperiod after image data is written by scanning a screen once, givenvoltage is continuously applied to liquid crystals; therefore, there isa problem in that liquid crystals are deteriorated and the image displayfunction is suppressed. Further, if the image data remains, the imagedata may remain on a screen even after the power source of the displaydevice is turned off.

Accordingly, an object of one embodiment of the present invention is tosuppress the above-described deterioration of the image display functionin a liquid crystal display device.

Another object of one embodiment of the present invention is to providea liquid crystal display device in which power consumption can bereduced and a driving method of the liquid crystal display device.

A liquid crystal display device operates when the supply of electricpower is started and does not operate when the supply of electric poweris stopped. In this specification, a state where electric power issupplied to the liquid crystal display device (a state where a powersource is ON) is referred to as an on state, and a state where thesupply of electric power is stopped (a state where a power source isOFF) is referred to as an off state. A control signal for turning on theliquid crystal display device is referred to as a start signal, and acontrol signal for turning off the liquid crystal display device isreferred to as a stop signal.

A liquid crystal element provided in the liquid crystal display deviceincludes a pixel electrode, a common electrode, and liquid crystalsprovided between the pixel electrode and the common electrode. Byapplying different potentials to the pixel electrode and the commonelectrode, voltage is applied to the liquid crystal element. Whenvoltage is applied to the liquid crystal element, electric field isgenerated and applied to the liquid crystals; therefore, the liquidcrystals respond, so that an image is displayed.

On the other hand, when the same potential is applied to the pixelelectrode and the common electrode, the potential difference does notoccur between the electrodes, whereby voltage is not applied to theliquid crystal element. Therefore, electric field is not generated inthe liquid crystal element and is not applied to the liquid crystals,whereby the liquid crystals do not respond. In this specification, astate of liquid crystals to which electric field is not applied (anon-response state) is referred to as an initial state (a liquid crystalinitial state).

In the display device which is in the on state by the supply of thestart signal, electric field is applied to the liquid crystals which isin the initial state; therefore, the liquid crystals respond and animage is displayed. Then, in the display device in the off state by thesupply of the stop signal, the liquid crystals return to the initialstate.

A liquid crystal display device disclosed in this specification has apixel structure in which a charge is accumulated in a capacitor andvoltage applied to the liquid crystals is maintained by the charge,whereby a display image is maintained. In the on state of the aboveliquid crystal display device, a semiconductor element whose currentvalue in the off state (an off-state current value) is low is preferablyused as a switching element which is electrically connected to thecapacitor and the liquid crystal element.

If the semiconductor element whose off-state current value is low isused as a switching element, the charge does not easily leak from thecapacitor through the semiconductor element, whereby voltage applied tothe liquid crystal element can be maintained for long period.Accordingly, a liquid crystal display device with high display-imagemaintaining property can be provided.

On the other hand, in a pixel in which the supply of electric power isstopped to be in the off state, the charge held by the capacitor needsto be completely discharged through the semiconductor element so thatthe liquid crystals to which electric field is applied and which are inthe response state can return to the initial state. In a period duringwhich the charge of the capacitor is discharged, electric field iscontinuously applied to the liquid crystals; therefore, the longer theperiod is, the more the deterioration of the liquid crystals isaccelerated. In a period during which the charge of the capacitor isdischarged, liquid crystals are responding and an image is maintained;therefore, in the case of a reflective liquid crystal display device inwhich external light is utilized as a light source, the image may remaineven after the power source is turned off (the image is seen as anafterimage), which is reduction in display quality.

As described above, if unnecessary electric field is continuouslyapplied to liquid crystals in the off state where an image is notdisplayed, the image display function and reliability as a liquidcrystal display device may be deteriorated.

In a liquid crystal display device disclosed in this specification, afixed potential is input to a capacitor before the power source isturned off, so that a potential difference between electrodes of thecapacitor disappears (capacitance becomes almost zero) such thatelectric field is not applied to the liquid crystals, whereby the liquidcrystals are in the initial state. Note that in this specification, animage displayed with liquid crystals in the initial state is referred toas an initial-state image. For example, in the case of a normally-whiteliquid crystal display device, the initial-state image is an all whiteimage, and in the case of a normally-black liquid crystal displaydevice, the initial-state image is an all black image. In the case ofthe normally-white liquid crystal display device, the initial-statedisplay can be a single-colored display with a color filter or a lightsource.

When the power source is turned off after the initial-state image isdisplayed, unnecessary electric field is not continuously applied to theliquid crystals in the off state, whereby the liquid crystals can be ina stable initial state.

Since the liquid crystal display device is in the off state afterdisplaying the initial-state image such as the all white image or theall black image, it can be prevented that image information displayedjust before the liquid crystal display device is turned off is leaked toothers, which occurs when an afterimage or the like is displayed on thescreen.

Accordingly, a liquid crystal display device in which favorable imagedisplay function can be maintained for long period and security is highcan be provided.

An embodiment of a driving method of a liquid crystal display devicedisclosed in this specification includes: displaying an image on ascreen provided with pixels each including a capacitor, a liquid crystalelement, and a semiconductor element by supplying a power sourcepotential from a power source and causing a liquid crystals of theliquid crystal element to respond; supplying a stop signal by a stopunit; displaying an initial-state image on the screen by writing a fixedpotential to the capacitor of each of the pixels in accordance with thestop signal and changing a state of the liquid crystals from a responsestate to a non-response state; and stopping the supply of the powersource potential from the power source.

An embodiment of a driving method of a liquid crystal display devicedisclosed in this specification includes: displaying an image on ascreen provided with pixels each including a capacitor, a liquid crystalelement, and a semiconductor element by supplying a power sourcepotential from a power source to a driver circuit portion and causing aliquid crystal of the liquid crystal element to respond; supplying astop signal by a stop unit; displaying an initial-state image on thescreen by writing a fixed potential to the capacitor of each of thepixels in accordance with the stop signal and changing a state of theliquid crystals from a response state to a non-response state; andstopping the supply of the power source potential from the power sourceto the driver circuit portion.

An embodiment of a driving method of a liquid crystal display devicedisclosed in this specification includes: displaying an image on ascreen provided with pixels each including a capacitor, a liquid crystalelement, and a semiconductor element by supplying a power sourcepotential from a power source to a driver circuit portion and abacklight portion and causing a liquid crystal of the liquid crystalelement to respond; supplying a stop signal by a stop unit; stopping thesupply of the power source potential from the power source to thebacklight portion; displaying an initial-state image on the screen bywriting a fixed potential to the capacitor of each of the pixels inaccordance with the stop signal and changing a state of the liquidcrystals from a response state to a non-response state; and stopping thesupply of the power source potential from the power source to the drivercircuit portion.

An embodiment of a driving method of a liquid crystal display devicedisclosed in this specification includes: displaying an image on ascreen provided with pixels each including a capacitor, a liquid crystalelement, and a semiconductor element by supplying a power sourcepotential from a power source to a driver circuit portion and abacklight portion and causing a liquid crystal of the liquid crystalelement to respond; supplying a stop signal by a stop unit; displayingan initial-state image on the screen by writing a fixed potential to thecapacitor of each of the pixels in accordance with the stop signal andchanging a state of the liquid crystals from a response state to anon-response state; and stopping the supply of the power sourcepotential from the power source to the driver circuit portion and thebacklight portion.

In the above-described structure, a transistor including an oxidesemiconductor layer can be used as a semiconductor element functioningas a switching element that is electrically connected to a capacitor anda liquid crystal element.

Before the liquid crystal display device is in the off state, the fixedpotential is written such that voltage is not applied to the liquidcrystal element, and an initial-state image is displayed. Accordingly,the liquid crystal element can be prevented from deteriorating,favorable image display function can be maintained for long period, andsecurity can be improved.

As a result, a liquid crystal display device with high reliability andlow power consumption can be achieved.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 is a diagram illustrating one embodiment of a liquid crystaldisplay device;

FIG. 2 is a diagram illustrating one embodiment of a liquid crystaldisplay device;

FIG. 3 is a diagram illustrating one embodiment of a liquid crystaldisplay device;

FIG. 4 is a timing chart illustrating one embodiment of a driving methodof a liquid crystal display device;

FIGS. 5A and 5B are timing charts illustrating one embodiment of adriving method of a liquid crystal display device;

FIG. 6 is a diagram illustrating one embodiment of a driving method of aliquid crystal display device;

FIGS. 7A to 7D are diagrams each illustrating one embodiment of atransistor which can be applied to a liquid crystal display device;

FIGS. 8A to 8E illustrate one embodiment of a method for manufacturing atransistor applicable to a liquid crystal display device;

FIGS. 9A and 9B are a block diagram and a diagram illustrating oneembodiment of a liquid crystal display device;

FIGS. 10A to 10F are diagrams illustrating an electronic device;

FIG. 11 is a diagram illustrating one embodiment of a liquid crystaldisplay device;

FIGS. 12A and 12B are pictures of display images of a liquid crystaldisplay device; and

FIGS. 13A and 13B are pictures of display images of a liquid crystaldisplay device.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, embodiments of the present invention will be described indetail with reference to the accompanying drawings. However, the presentinvention is not limited to the description below, and it is easilyunderstood by those skilled in the art that embodiments and detailsdisclosed herein can be modified in various ways. In addition, thepresent invention is not construed as being limited to description ofthe embodiments.

Embodiment 1

In this embodiment, one embodiment of a liquid crystal display deviceand that of a driving method of a liquid crystal display device aredescribed with reference to FIG. 1 and FIG. 2.

The liquid crystal display device of this embodiment will be describedwith reference to a flow chart of FIG. 1.

As illustrated in FIG. 1, an image A is displayed on a display screen ofthe liquid crystal display device. When another display image obtainedby the supply of another image signal is not needed (when use of theliquid crystal display device is finished), a stop unit is selected.After the stop unit is selected, a stop signal is input and a fixedpotential is written to capacitors of all the pixels. By writing thefixed potential to the capacitor, a potential difference betweenelectrodes of the capacitor disappears (in other words, capacitancebecomes almost zero), whereby liquid crystals in a response state areswitched to the initial state of the non-response state. Accordingly, aninitial-state image S that is displayed by the liquid crystals in theinitial state is displayed on the display screen. For example, in thecase of a normally-white liquid crystal display device, theinitial-state image S is displayed in all white, and in the case of anormally-black liquid crystal display device, the initial-state image Sis displayed in all black. In the case of the normally-white liquidcrystal display device, the initial-state display can be asingle-colored display with a color filter or a light source.

After the initial-state image S is displayed, a power source is stoppedand the supply of a power source potential to a display panel isstopped, whereby the liquid crystal display device is in the off state.Therefore, unnecessary electric field is not continuously applied to theliquid crystals in the off state, whereby the liquid crystals can be ina stable initial state.

Since the liquid crystal display device is in the off state afterdisplaying the initial-state image such as the all white image or theall black image, it can be prevented that image information displayedjust before the liquid crystal display device is turned off is leaked toothers, which occurs when an afterimage or the like is displayed on thescreen.

Therefore, a liquid crystal display device in which favorable imagedisplay function can be maintained for long period and security is highcan be provided.

Each configuration of a liquid crystal display device 100 of thisembodiment will be described with reference to a block diagram of FIG.2. The liquid crystal display device 100 includes a power source 116, astop unit 117, a display control circuit 113, and a display panel 120.In the case of a transmissive liquid crystal display device or asemi-transmissive liquid crystal display device, a backlight portion asa light source may be also provided.

To the liquid crystal display device 100, an image signal (an imagesignal Data) is supplied from an external device connected to the liquidcrystal display device. A power source potential (a high power sourcepotential V_(dd), a low power source potential V_(ss), and a commonpotential V_(com)) is supplied when the power source 116 of the liquidcrystal display device is in the on state and the power supply isstarted. The control signal (a start pulse SP and a clock signal CK) issupplied from the display control circuit 113. The supply of the powersource potential (the high power source potential V_(dd), the low powersource potential V_(ss), and the common potential V_(com)) is stopped bythe control of the stop unit 117. After the initial-state image isdisplayed, the power source 116 is turned off, so that the supply of thepower source potential to the display panel is stopped.

Note that the high power source potential V_(dd) is a potential higherthan a reference potential, and the low power source potential V_(ss) isa potential lower than or equal to the reference potential. Note that itis desirable that each of the high power source potential V_(dd) and thelow power source potential V_(ss) be a potential such that a transistorcan operate. Note that the difference between the high power sourcepotential V_(dd) and the low power source potential V_(ss) is referredto as a power source voltage in some cases.

The common potential V_(com) may be any fixed potential as long as itserves as reference with respect to a potential of an image signal Datasupplied to a pixel electrode. For example, the common potential V_(com)may be a ground potential.

Note that the image signal Data may be appropriately inverted inaccordance with dot inversion driving, source line inversion driving,gate line inversion driving, frame inversion driving, or the like to beinput to the liquid crystal display device 100. In the case where animage signal Data is an analog signal, the image signal Data may beconverted into a digital signal through an A/D converter or the like tobe supplied to the liquid crystal display device 100.

In this embodiment, a common electrode 128 and one electrode of acapacitor 210 are provided with the common potential V_(com) that is afixed potential through the display control circuit 113 from the powersource 116.

The display control circuit 113 is a circuit which supplies a displaypanel image signal (Data), the control signal (specifically, the startpulse SP, the clock signal CK, and the like), and the power sourcepotential (the high power source potential V_(dd), the low power sourcepotential V_(ss), and the common potential V_(com)) to the display panel120.

The display panel 120 has a structure in which a liquid crystal element215 is sandwiched between a pair of substrate (a first substrate and asecond substrate). The first substrate is provided with a driver circuitportion 121 and a pixel portion 122. The second substrate is providedwith a common connection portion (also referred to as a common contact)and the common electrode 128 (also referred to as a counter electrode).The common connection portion electrically connects a first substrateand a second substrate. The common connection portion may be providedover the first substrate.

In the pixel portion 122, a plurality of gate lines (scan lines) 124 anda plurality of source lines (signal lines) 125 are provided. A pluralityof pixels 123 is arranged in matrix so that each of pixels 123 issurrounded by the gate lines 124 and the source lines 125. In thedisplay panel described in this embodiment, the gate lines 124 and thesource lines 125 are extended from a gate line driver circuit 121A and asource line driver circuit 121B, respectively.

In addition, the pixel 123 includes a transistor 214 as a switchingelement, the capacitor 210 connected to the transistor 214, and theliquid crystal element 215.

The liquid crystal element 215 is an element that controls transmissionand non-transmission of light by the optical modulation action of liquidcrystals. The optical modulation action of liquid crystals is controlledby electric field applied to the liquid crystals. The direction of theelectric field applied to the liquid crystals varies according to aliquid crystal material, a driving method, and an electrode structureand is selected as appropriate. For example, in the case where a drivingmethod in which electric field is applied in a thickness direction (aso-called vertical direction) of a liquid crystal layer is used, thefirst substrate and the second substrate may be provided with the pixelelectrode and the common electrode, respectively, with the liquidcrystals provided between the first substrate and the second substrate.In the case where a driving method in which electric field is applied inan in-plane direction (a so-called horizontal direction) is used, thepixel electrode and the common electrode may be provided on the samesubstrate with respect to the liquid crystals. The pixel electrode andthe common electrode may have a variety of opening patterns. In thisembodiment, there is no particular limitation on a liquid crystalmaterial, a driving method, and an electrode structure as long as anelement controls transmission and non-transmission of light by theoptical modulation action.

In the transistor 214, one of the plurality of gate lines 124 providedin the pixel portion 122 is connected to the gate electrode, one of asource electrode and a drain electrode is connected to one of theplurality of source lines 125, and the other of the source electrode andthe drain electrode is connected to one of the electrodes of thecapacitor 210 and one of the electrodes of the liquid crystal element215 (pixel electrode).

A transistor having low off-state current is preferably used for thetransistor 214. When the transistor 214 is in the off state, electriccharges accumulated in the liquid crystal element 215 and the capacitor210 which are connected to the transistor 214 having low off-statecurrent are hardly leaked through the transistor 214, so that the statewhere data is written before the transistor 214 is switched to the offstate can be maintained for long time.

With such a structure, the capacitor 210 can hold voltage applied to theliquid crystal element 215. The electrode of the capacitor 210 may beconnected to a capacitor line additionally provided.

The driver circuit portion 121 includes the gate line driver circuit121A and the source line driver circuit 121B. The gate line drivercircuit 121A and the source line driver circuit 121B are driver circuitsfor driving the pixel portion 122 that includes the plurality of pixelsand each include a shift register circuit (also referred to as a shiftregister).

Note that the gate line driver circuit 121A and the source line drivercircuit 121B may be formed over the same substrate as the pixel portion122 or over a different substrate from the pixel portion 122.

Note that the high power source potential V_(dd), the low power sourcepotential V_(ss), the start pulse SP, the clock signal CK, and the imagesignal Data which are controlled by the display control circuit 113 aresupplied to the driver circuit portion 121.

A terminal portion 126 is an input terminal which supplies apredetermined signal output from the display control circuit 113 (suchas the high power source potential V_(dd), the low power sourcepotential V_(ss), the start pulse SP, the clock signal CK, the imagesignal Data, and the common potential V_(com)) and the like, to thedriver circuit portion 121.

The common electrode 128 is electrically connected to a common potentialline for supplying the common potential V_(com) which is controlled bythe display control circuit 113, in the common connection portion.

As a specific example of the common connection portion, the commonelectrode 128 and the common potential line can be electricallyconnected with a conductive particle in which an insulating sphere iscovered with a thin metal film provided therebetween. Note that two ormore common connection portions may be provided in the display panel120.

The liquid crystal display device may include a photometric circuit. Theliquid crystal display device provided with the photometric circuit candetect the brightness of the environment where the liquid crystaldisplay device is put. As a result, the display control circuit 113connected to the photometric circuit can control a driving method of alight source such as a backlight and a sidelight in accordance with asignal input from the photometric circuit.

Color display can be performed by a combination with color filters.Also, other optical films (such as a polarizing film, a retardationfilm, or an anti-reflection film) can be used in combination. A lightsource such as a backlight that is used in a transmissive liquid crystaldisplay device or a semi-transmissive liquid crystal display device maybe selected and combined in accordance with the use of the liquidcrystal display device 100. Further also, a planar light source may beformed using a plurality of LED light sources or a plurality ofelectroluminescent (EL) light sources. As the planar light source, threeor more kinds of LEDs may be used and an LED emitting white light may beused. Note that the color filter is not always provided in the casewhere light-emitting diodes of RGB or the like are arranged in abacklight and a successive additive color mixing method (a fieldsequential method) in which color display is performed by time divisionis employed.

As described above, in the on state where the liquid crystal displaydevice is turned on and electric power is supplied, low powerconsumption can be achieved with the use of a semiconductor elementhaving low off-state current. Before the liquid crystal display deviceis in the off state, the fixed potential is written such that voltage isnot applied to the liquid crystal element, and an initial-state image isdisplayed; therefore the liquid crystal element can be prevented fromdeteriorating, favorable image display function can be maintained forlong period, and security can be improved.

Accordingly, a highly reliable liquid crystal display device in whichlow power consumption is achieved and a driving method of the liquidcrystal display device can be provided.

Embodiment 2

In this embodiment, a driving method of a liquid crystal display devicein which low power consumption can be achieved by a combination withEmbodiment 1. The same portions as Embodiment 1 or portions havingfunctions similar to those described in Embodiment 1 can be formed in amanner similar to that described in Embodiment 1; therefore, repetitivedescription is omitted. In addition, detailed description of the sameportions is not repeated.

A liquid crystal display device displays images on a screen in acombination with a moving image and a still image. By switching of aplurality of different images corresponding to a plurality of frames athigh speed, the images are recognized as the moving image by human eyes.Specifically, by switching of images at least 60 times (60 frames) persecond, the images are recognized as a moving image with less flicker byhuman eyes. In contrast, unlike a moving image and a partial movingimage, a still image is an image which does not change in successiveframe periods, for example, in an n-th frame and an (n+1)th frame, whena plurality of images corresponding to a plurality of frame periods fortime division are switched at high speed.

The liquid crystal display device according to the present invention canbe operated in different display modes, a moving-image display mode anda still-image display mode, in the case of displaying a moving image anddisplaying a still image, respectively. In this specification, an imagedisplayed in the still-image display mode is referred to as a stillimage.

In the case of displaying a moving image in which image signals in aseries of frames are different (e.g., a first image signal correspondingto a first frame and a second image signal corresponding to a secondframe are different to each other when the first frame and the secondframe are successive frames), a display mode in which an image signal iswritten in each frame is used. In the case of displaying a still imagein which image signals in a series of frames are the same (e.g., a firstimage signal corresponding to a first frame and a second image signalcorresponding to a second frame are the same when the first frame andthe second frame are successive frames), another image signal is notwritten, and a display mode in which a still image is displayed in sucha way that potentials of the pixel electrode and the common electrodewhich apply voltage to the liquid crystal element are set to be in afloating state to maintain voltage applied to the liquid crystalelement, so that a still image is displayed without supply of anotherpotential.

The liquid crystal display device of this embodiment and a switchingbetween the moving-image display mode and the still-image display modeof the liquid crystal display device will be described with reference toFIG. 3, FIG. 4, FIGS. 5A and 5B, FIG. 6, and FIG. 11.

Each configuration of a liquid crystal display device 200 of thisembodiment will be described with reference to a block diagram of FIG.11. The liquid crystal display device 200 is an example of atransmissive liquid crystal display device or a semi-transmissive liquidcrystal display device in which display is performed by utilizingtransmission or non-transmission of light in pixels. The liquid crystaldisplay device 200 includes an image processing circuit 110, a powersource 116, a stop unit 117, a display panel 120, and a backlightportion 130. In the case of a reflective liquid crystal display device,external light is utilized as a light source; therefore, the backlightportion 130 can be omitted.

To the liquid crystal display device 200, an image signal (an imagesignal Data) is supplied from an external device connected to the liquidcrystal display device. Note that a power source potential (a high powersource potential V_(dd), a low power source potential V_(ss), and acommon potential V_(com)) is supplied when the power source 116 of theliquid crystal display device is turned on and the power supply isstarted. The control signal (a start pulse SP and a clock signal CK) issupplied from the display control circuit 113. The supply of the powersource potential (the high power source potential V_(dd), the low powersource potential V_(ss), and the common potential V_(com)) is stopped bythe control of the stop unit 117. After the initial-state image isdisplayed, the power source 116 is turned off, so that the supply of thepower source potential to the display panel is stopped.

In the case where the image signal Data is an analog signal, the imagesignal is preferably converted into a digital signal through an A/Dconverter or the like to be supplied to the image processing circuit 110of the liquid crystal display device 200; because, when a difference ofimage signals is detected later, the difference can be detected easily.

A configuration of the image processing circuit 110 and a process inwhich the image processing circuit 110 processes a signal will bedescribed.

The image processing circuit 110 includes a memory circuit 111, acomparison circuit 112, a display control circuit 113, and a selectioncircuit 115. The image processing circuit 110 generates a display-panelimage signal and a backlight signal from the digital image signal Datathat is input. The display-panel image signal is an image signal thatcontrols the display panel 120. The backlight signal is a signal thatcontrols the backlight portion 130. The image processing circuit 110outputs a signal that controls a common electrode 128 to a switchingelement 127.

The memory circuit 111 includes a plurality of frame memories forstoring image signals of a plurality of frames. The number of framememories included in the memory circuit 111 is not particularly limitedas long as the image signals of a plurality of frames can be stored.Note that the frame memory may be formed using a memory element such asdynamic random access memory (DRAM) or static random access memory(SRAM).

The number of frame memories is not particularly limited as long as theimage signal can be stored for each frame period. Further, the imagesignals stored in the frame memories are selectively read out by thecomparison circuit 112 and the display control circuit 113. A framememory 111 b in the diagram illustrates a memory region for one frameconceptually.

In one of these frame memories, an image signal of the initial-stateimage (e.g., an all white image or an all black image) in which liquidcrystals are switched to the initial state of the non-response state,which is described in Embodiment 1, can be stored. The image signal ofthe initial-state image is read out by the display control circuit 113when the stop signal is input, so that the image signal of theinitial-state image is written to the screen.

The comparison circuit 112 is a circuit that selectively reads out imagesignals in successive frame periods stored in the memory circuit 111,compares the image signals in the successive frame periods in eachpixel, and detects a difference thereof.

In this embodiment, depending on whether a difference of image signalsbetween frames is detected or not, operations in the display controlcircuit 113 and the selection circuit 115 are determined. When adifference is detected in any of the pixels between frames by thecomparison circuit 112 (when there is a difference), the comparisoncircuit 112 determines that the image signal is not a signal fordisplaying a still image and successive frame periods during which adifference is detected is a period during which a moving image is to bedisplayed.

On the other hand, when a difference is not detected in any of thepixels by comparing the image signals in the comparison circuit 112(when there is no difference), the successive frame periods during whichthe difference is not detected is determined as period during which astill image is to be displayed. In other words, by detection of thedifferences in the comparison circuit 112, the image signals insuccessive frame periods are determined as image signals for displayingmoving images or image signals for displaying still images.

Note that the criterion of determining that there is a difference by thecomparison may be set such that the difference is recognized when thedifference detected by the comparison circuit 104 exceeds a certainvalue. The comparison circuit 112 may be set to determine detection of adifference by the absolute value of the difference.

Although, in this embodiment, the structure in which an image isdetermined to be a moving image or a still image by detection of thedifference between the image signals in successive frame periods by thecomparison circuit 112 provided inside the liquid crystal display device200 is described, a structure in which a signal as to whether the imageis a still image or a moving image is supplied from the outside may beused.

The selection circuit 115 includes a plurality of switches, for example,switches formed using transistors. In the case where the comparisoncircuit 112 detects a difference in successive frame periods, that is,the image is a moving image, the selection circuit 115 selects an imagesignal of the moving image from the frame memories in the memory circuit111 and outputs the image signal to the display control circuit 113.

Note that in the case where the comparison circuit 112 does not detect adifference in the successive frame periods, that is, the image is astill image, the selection circuit 115 does not output the image signalto the display control circuit 113 from the frame memories in the memorycircuit 111. With the structure in which an image signal is not outputto the display control circuit 113 from the frame memory, powerconsumption of the liquid crystal display device can be reduced.

Note that in the liquid crystal display device of this embodiment, amode performed in such a way that the comparison circuit 112 determinesthe image signals as a still image is described as the still-imagedisplay mode, and a mode performed in such a way that the comparisoncircuit 112 determines the image signals as a moving image is describedas the moving-image display mode.

The display control circuit 113 is a circuit which supplies an imagesignal selected in the selection circuit 115, the control signal(specifically, a signal for controlling switching between supply andstop of the control signal such as the start pulse SP and the clocksignal CK), and the power source potential (the high power sourcepotential V_(dd), the low power source potential V_(ss), and the commonpotential V_(com)) to the display panel 120 and which supplies abacklight control signal (specifically, a signal in which the backlightcontrol circuit 131 controls on and off of a backlight) to the backlightportion 130.

Note that the image processing circuit described in this embodiment asan example may have a display-mode switching function. The display-modeswitching function is a function of switching between a moving-imagedisplay mode and a still-image display mode in such a manner that a userof the liquid crystal display device selects an operation mode of theliquid crystal display device by hand or using an external connectiondevice.

The selection circuit 115 can output the image signal to the displaycontrol circuit 113 in accordance with a signal input from adisplay-mode switching circuit.

For example, in the case where a mode-switching signal is input to theselection circuit 115 from the display-mode switching circuit whileoperation is performed in a still-image display mode, even when thecomparison circuit 112 does not detect the difference of the imagesignals in successive frame periods, the selection circuit 115 can beoperated in a mode in which image signals which are input aresequentially output to the display control circuit 113, that is, in amoving-image display mode. In the case where a mode-switching signal isinput to the selection circuit 115 from the display-mode switchingcircuit while operation is performed in a moving-image display mode,even when the comparison circuit 112 detects the difference of the imagesignal in successive frame periods, the selection circuit 115 can beoperated in a mode in which only an image signal of one selected frameis output, that is, in a still-image display mode. Therefore, when theliquid crystal display device of this embodiment is operated in themoving-image display mode, one image corresponding to one frame amongimages corresponding to a plurality of frames for time division isdisplayed as a still image.

The liquid crystal display device may include a photometric circuit. Theliquid crystal display device provided with the photometric circuit candetect the brightness of the environment where the liquid crystaldisplay device is put. As a result, the display control circuit 113connected to the photometric circuit can control a driving method of alight source such as a backlight in accordance with a signal input fromthe photometric circuit.

For example, when the photometric circuit detects the liquid crystaldisplay device is used in a dim environment, the display control circuit113 controls the intensity of light from the backlight 132 to beincreased so that visibility of the display screen is improved. Incontrast, when the photometric circuit detects the liquid crystaldisplay device is used under extremely bright external light (e.g.,under direct sunlight outdoors), the display control circuit 113controls the intensity of light from the backlight 132 to be lowered sothat power consumption of the backlight 132 is reduced.

The backlight portion 130 includes the backlight control circuit 131 andthe backlight 132. The backlight 132 may be selected and combined inaccordance with the use of the liquid crystal display device 200. As alight source of the backlight 132, a cold cathode fluorescent lamp or alight-emitting diode (LED) can be used. Color display can be performedby a combination with color filters. For example, white light-emittingelement (e.g., LED) can be arranged in the backlight 132. Note that thecolor filter is not always provided in the case where light-emittingdiodes of RGB or the like are arranged in the backlight 132 and asuccessive additive color mixing method (a field sequential method) inwhich color display is performed by time division is employed. Abacklight signal for controlling the backlight and the power sourcepotential are supplied from the display control circuit 113 to thebacklight control circuit 131.

In this embodiment, the display panel 120 includes the switching element127 besides the pixel portion 122. In this embodiment, the display panel120 includes a first substrate and a second substrate. The firstsubstrate is provided with a driver circuit portion 121, the pixelportion 122, and the switching element 127.

The pixel 123 includes the transistor 214 as a switching element, acapacitor 210, and a liquid crystal element 215, which are connected tothe transistor 214 (see FIG. 3).

A transistor having low off-state current is preferably used for thetransistor 214. When the transistor 214 is in the off state, electriccharges accumulated in the liquid crystal element 215 and the capacitor210 which are connected to the transistor 214 having low off-statecurrent are hardly leaked through the transistor 214, so that the statewhere data is written before the transistor 214 is in the off state canbe maintained for long time.

In this embodiment, liquid crystals are controlled by a verticalelectric field that is generated by the pixel electrode over the firstsubstrate and the common electrode provided on the second substratewhich face to the first substrate.

As an example of a liquid crystal applied to a liquid crystal element,the following can be used: a nematic liquid crystal, a cholestericliquid crystal, a smectic liquid crystal, a discotic liquid crystal, athermotropic liquid crystal, a lyotropic liquid crystal, a low-molecularliquid crystal, a high-molecular liquid crystal, a polymer dispersedliquid crystal (PDLC), a ferroelectric liquid crystal, ananti-ferroelectric liquid crystal, a main-chain liquid crystal, aside-chain high-molecular liquid crystal, a plasma addressed liquidcrystal (PALC), a banana-shaped liquid crystal, and the like.

In addition, as a driving method of a liquid crystal, the following canbe used: a TN (twisted nematic) mode, an STN (super twisted nematic)mode, an OCB (optically compensated birefringence) mode, an ECB(electrically controlled birefringence) mode, an FLC (ferroelectricliquid crystal) mode, an AFLC (anti-ferroelectric liquid crystal) mode,a PDLC (polymer dispersed liquid crystal) mode, a PNLC (polymer networkliquid crystal) mode, a guest-host mode, and the like.

The switching element 127 supplies a common potential V_(com) to thecommon electrode 128 in accordance with a control signal output from thedisplay control circuit 113. As the switching element 127, a transistorcan be used. A gate electrode and one of a source electrode and a drainelectrode of the transistor may be connected to the display controlcircuit 113, the common potential V_(com) may be supplied from thedisplay control circuit 113 to the one of the source electrode and thedrain electrode through the terminal portion 126, and the other thereofmay be connected to the common electrode 128. Note that the switchingelement 127 may be formed over the substrate over which the drivercircuit portion 121 or the pixel portion 122 is formed, or over adifferent substrate from them.

Since a transistor having low off-state current is used for theswitching element 127, a decrease over time of the voltage applied toboth terminals of the liquid crystal element 215 can be suppressed.

In the common connection portion, a terminal connected to a sourceelectrode or a drain electrode of the switching element 127 and thecommon electrode 128 is electrically connected to each other.

One of the source electrode and the drain electrode of the switchingelement 127 for which a transistor that is one embodiment of a switchingelement is used is connected to one electrode of the capacitor 210 andone electrode of the liquid crystal element 215 which are not connectedto the transistor 214, and the other of the source electrode and thedrain electrode of the switching element 127 is connected to a terminal126B. A gate electrode of the switching element 127 is connected to aterminal 126A.

Next, the state of signals supplied to the pixels will be described withreference to FIG. 3 illustrating an equivalent circuit diagram of theliquid crystal display device and FIG. 4 illustrating a timing chart.

In FIG. 4, a clock signal GCK and a start pulse GSP which are suppliedfrom the display control circuit 113 to the gate line driver circuit121A are illustrated. In addition, in FIG. 4, a clock signal SCK and astart pulse SSP which are supplied from the display control circuit 113to the source line driver circuit 121B are illustrated. To describe anoutput timing of the clock signal, the waveform of the clock signal isindicated with simple rectangular wave in FIG. 4.

In FIG. 4, a potential of a source line (Data line) 125, a potential ofa pixel electrode, a potential of the terminal 126A, a potential of theterminal 126B, and a potential of a common electrode are illustrated.

In FIG. 4, a period 1401 corresponds to a period during which imagesignals for displaying a moving image are written. In the period 1401,operation is performed so that the image signals and the commonpotential are supplied to the pixels in the pixel portion 122 and thecommon electrode.

A period 1402 corresponds to a period during which a still image isdisplayed. In the period 1402, the supply of the image signals to thepixels in the pixel portion 122 and the supply of the common potentialto the common electrode are stopped. Note that each signal for stoppingthe operation of the driver circuit portion is supplied in the period1402 illustrated in FIG. 4; however, it is preferable to preventdeterioration of a still image by writing image signals periodically inaccordance with the length of the period 1402 and a refresh rate.

First, a timing chart in the period 1401 will be described. In theperiod 1401, a clock signal is supplied all the time as the clock signalGCK, and a pulse in accordance with a vertical synchronizing frequencyis supplied as the start pulse GSP. In the period 1401, a clock signalis supplied all the time as the clock signal SCK, and a pulse inaccordance with one gate selection period is supplied as the start pulseSSP.

An image signal Data is supplied to pixels in each row through thesource line 125, and a potential of the source line 125 is supplied tothe pixel electrode in accordance with a potential of a gate line 124.

A potential at which the switching element 127 is turned on is suppliedfrom the display control circuit 113 to the terminal 126A of theswitching element 127, so that a common potential is supplied to thecommon electrode through the terminal 126B.

On the other hand, the period 1402 is a period during which a stillimage is displayed. Next, a timing chart in the period 1402 isdescribed. In the period 1402, supplies of the clock signal GCK, thestart pulse GSP, the clock signal SCK, and the start pulse SSP are allstopped. In addition, the supply of the image signal Data to the sourceline 125 is stopped in the period 1402. In the period 1402 during whichsupplies of the clock signal GCK and the start pulse GSP are stopped,the transistor 214 is turned off and a potential of the pixel electrodeis put in the floating state.

A potential at which the switching element 127 is turned off is suppliedfrom the display control circuit 113 to the terminal 126A of theswitching element 127, so that a potential of the common electrode isput in the floating state.

In the period 1402, both electrodes of the liquid crystal element 215,i.e., the pixel electrode and the common electrode, are put in thefloating state; thus, a still image can be displayed without the supplyof another potential.

The supplies of the clock signal and the start pulse to the gate linedriver circuit 121A and the source line driver circuit 121B are stopped,whereby low power consumption can be achieved.

In particular, when transistors having low off-state current are usedfor the transistor 214 and the switching element 127, a decrease overtime of the voltage applied to both terminals of the liquid crystalelement 215 can be suppressed.

Next, operation of the display control circuit in a period during whicha display image is switched from a moving image to a still image (aperiod 1403 in FIG. 4), and a period during which a display image isswitched from a still image to a moving image (a period 1404 in FIG. 4)will be described with reference to FIGS. 5A and 5B. FIGS. 5A and 5Billustrate potentials of high power source potential V_(dd), the clocksignal (here, GCK), the start pulse signal (here, GSP) which are outputfrom the display control circuit, and a potential of the terminal 126A.

FIG. 5A illustrates operation of the display control circuit in theperiod 1403 during which a display image is switched from a moving imageto a still image. The display control circuit stops the supply of thestart pulse GSP (E1 in FIG. 5A, a first step). The supply of the startpulse GSP is stopped and then, the supply of a plurality of clocksignals GCK is stopped after pulse output reaches the last stage of theshift register (E2 in FIG. 5A, a second step). Then, the high powersource potential V_(dd) of a power source is changed to the low powersource potential V_(ss) (E3 in FIG. 5A, a third step). After that, thepotential of the terminal 126A is changed to a potential at which theswitching element 127 is turned off (E4 in FIG. 5A, a fourth step).

Through the above steps, the supply of the signals to the driver circuitportion 121 can be stopped without causing malfunction of the drivercircuit portion 121. The malfunction occurred when a display image isswitched from a moving image to a still image causes noise, and thenoise is held as a still image; therefore, a liquid crystal displaydevice that includes a display control circuit with fewer malfunctionscan display a still image which is not deteriorated so much.

Next, operation of the display control circuit in the period 1404 duringwhich a display image is switched from a still image to a moving imagewill be illustrated in FIG. 5B. The display control circuit sets apotential of the terminal 126A in a potential at which the switchingelement 127 is turned on (S1 in FIG. 5B, a first step). Then, a powersource voltage is changed from the low power source potential V_(ss) tothe high power source potential V_(dd) (S2 in FIG. 5B, a second step). Ahigh-level potential is applied as the clock signal GCK, after that aplurality of clock signals GCK is supplied (S3 in FIG. 5B, a thirdstep). Next, the start pulse signal GSP is supplied (S4 in FIG. 5B, afourth step).

Through the above steps, the supply of drive signals to the drivercircuit portion 121 can be resumed without causing malfunction of thedriver circuit portion 121. Potentials of wirings are sequentially setback to those at the time of displaying a moving image, the drivercircuit portion can be driven without malfunction.

FIG. 6 schematically illustrates writing frequency of image signal ineach frame period in a period 601 during which a moving image isdisplayed or in a period 602 during which a still image is displayed. InFIG. 6, “W” indicates a period during which an image signal is written,and “H” indicates a period during which the image signal is held. Inaddition, a period 603 is one frame period in FIG. 6; however, theperiod 603 may be a different period.

As described above, in the structure of the liquid crystal displaydevice of this embodiment, an image signal of a still image displayed inthe period 602 is written in the period 604, and the image signalwritten in the period 604 is maintained in the other period of theperiod 602.

The liquid crystal display device described in this embodiment as anexample can decrease writing frequency of an image signal in a periodduring which a still image is displayed. As a result, power consumptionat the time when a still image is displayed can be reduced.

In the case where a still image is displayed by rewriting the same imageplural times, visible switching of the images may cause fatigue of thehuman eye. In the liquid crystal display device of this embodiment,writing frequency of an image signal is decreased, which makes eyestrainless severe.

In particular, in the liquid crystal display device of this embodiment,a transistor having low off-state current is applied to each pixel and aswitching element for a common electrode, whereby a period (the lengthof time) in which a storage capacitor can maintain voltage can beextended. As a result, writing frequency of an image signal can beextremely reduced, whereby there is a significant effect in reducingpower consumption and eyestrain when a still image is displayed.

In the liquid crystal display device 200 of this embodiment, after thestop unit 117 is selected, a stop signal is input and a fixed potentialis written to the capacitor 210 of all the pixels. By writing the fixedpotential to the capacitors 210, a potential difference betweenelectrodes of the capacitors 210 disappears, whereby liquid crystals ina response state are switched to the initial state of the non-responsestate. Accordingly, an initial-state image that is displayed by theliquid crystals in the initial state is displayed on the display screen.

After the initial-state image is displayed, the power source 116 isturned off and the supply of a power source potential to the displaypanel 120 is stopped, whereby the liquid crystal display device 200 isturned off. Therefore, unnecessary electric field is not continuouslyapplied to liquid crystals in the off state, whereby the liquid crystalscan be in a stable initial state.

As described above, in the on state where the liquid crystal displaydevice is turned on and electric power is supplied, the moving-imagedisplay mode or the still-image display mode is selected as appropriatein accordance with image signals in successive frames, whereby low powerconsumption can be achieved. Further, before the liquid crystal displaydevice is in the off state, the fixed potential is written such thatvoltage is not applied to the liquid crystal element, and aninitial-state image is displayed; therefore the liquid crystal elementcan be prevented from deteriorating, favorable image display functioncan be maintained for long period, and security can be improved.

Accordingly, a highly reliable liquid crystal display device in whichlow power consumption is achieved and a driving method of the liquidcrystal display device can be provided.

Embodiment 3

In this embodiment, an example of a transistor that can be applied to aliquid crystal display device disclosed in this specification will bedescribed. There is no particular limitation on a structure of atransistor that can be applied to the liquid crystal display devicedisclosed in this specification. For example, a top-gate structure or abottom-gate structure such as a staggered type and a planar type can beused. Further, the transistor may have a single-gate structure includingone channel formation region, a double-gate structure including twochannel formation regions, or a triple-gate structure including threechannel formation regions. Alternatively, the transistor may have adual-gate structure including two gate electrode layers positioned overand below a channel region with a gate insulating layer providedtherebetween. FIGS. 7A to 7D illustrate an example of a cross-sectionalstructure of a transistor. Note that transistors illustrated in FIGS. 7Ato 7D are transistors including an oxide semiconductor as asemiconductor. An advantage of using an oxide semiconductor is that highmobility and low off-state current can be obtained in a relatively easyand low-temperature process; needless to say, another semiconductor maybe used.

A transistor 410 illustrated in FIG. 7A is a kind of bottom-gate thinfilm transistor and is also referred to as an inverted-staggered thinfilm transistor.

The transistor 410 includes, over a substrate 400 having an insulatingsurface, a gate electrode layer 401, a gate insulating layer 402, anoxide semiconductor layer 403, a source electrode layer 405 a, and adrain electrode layer 405 b. In addition, an insulating film 407 thatcovers the transistor 410 and is stacked over the oxide semiconductorlayer 403 is provided. A protective insulating layer 409 is formed overthe insulating film 407.

A transistor 420 illustrated in FIG. 7B is one of bottom-gate thin filmtransistors referred to as a channel-protective type (channel-stop type)and is also referred to as an inverted-staggered thin film transistors.

The transistor 420 includes, over the substrate 400 having an insulatingsurface, the gate electrode layer 401, the gate insulating layer 402,the oxide semiconductor layer 403, an insulating layer 427 thatfunctions as a channel protective layer covering a channel formationregion of the oxide semiconductor layer 403, the source electrode layer405 a, and the drain electrode layer 405 b. The protective insulatinglayer 409 is provided to cover the transistor 420.

A transistor 430 illustrated in FIG. 7C is a bottom-gate thin filmtransistor and includes, over the substrate 400 having an insulatingsurface, the gate electrode layer 401, the gate insulating layer 402,the source electrode layer 405 a, the drain electrode layer 405 b, andthe oxide semiconductor layer 403. The insulating film 407 which coversthe transistor 430 and is in contact with the oxide semiconductor layer403 is provided. The protective insulating layer 409 is formed over theinsulating film 407.

In the transistor 430, the gate insulating layer 402 is provided on andin contact with the substrate 400 and the gate electrode layer 401. Thesource electrode layer 405 a and the drain electrode layer 405 b areprovided on and in contact with the gate insulating layer 402. The oxidesemiconductor layer 403 is provided over the gate insulating layer 402,the source electrode layer 405 a, and the drain electrode layer 405 b.

A transistor 440 illustrated in FIG. 7D is one of top-gate thin filmtransistors. The transistor 440 includes, over the substrate 400 havingan insulating surface, an insulating layer 437, the oxide semiconductorlayer 403, the source electrode layer 405 a, the drain electrode layer405 b, the gate insulating layer 402, and the gate electrode layer 401.A wiring layer 436 a and a wiring layer 436 b are provided to be incontact with and electrically connected to the source electrode layer405 a and the drain electrode layer 405 b, respectively.

In this embodiment, as described above, the oxide semiconductor layer403 is used as a semiconductor layer. As an oxide semiconductor used forthe oxide semiconductor layer 403, an In—Sn—Ga—Zn—O-based oxidesemiconductor which is an oxide of four metal elements; anIn—Ga—Zn—O-based oxide semiconductor, an In—Sn—Zn—O-based oxidesemiconductor, an In—Al—Zn—O-based oxide semiconductor, aSn—Ga—Zn—O-based oxide semiconductor, an Al—Ga—Zn—O-based oxidesemiconductor, or a Sn—Al—Zn—O-based oxide semiconductor which areoxides of three metal elements; an In—Zn—O-based oxide semiconductor, aSn—Zn—O-based oxide semiconductor, an Al—Zn—O-based oxide semiconductor,a Zn—Mg—O-based oxide semiconductor, a Sn—Mg—O-based oxidesemiconductor, or an In—Mg—O-based oxide semiconductor which are oxidesof two metal elements; an In—O-based oxide semiconductor, a Sn—O-basedoxide semiconductor, or a Zn—O-based oxide semiconductor can be used.Further, SiO₂ may be contained in the above oxide semiconductor. Here,for example, the In—Ga—Zn—O-based oxide semiconductor means an oxidecontaining at least In, Ga, and Zn, and the composition ratio of theelements is not particularly limited. The In—Ga—Zn—O-based oxidesemiconductor may contain an element other than In, Ga, and Zn.

For the oxide semiconductor layer 403, a thin film represented by thechemical formula, InMO₃(ZnO)_(m) (m>0), can be used. Here, M representsone or more metal elements selected from Ga, Al, Mn, and Co. Forexample, M can be Ga, Ga and Al, Ga and Mn, Ga and Co, or the like.

In each of the transistors 410, 420, 430, and 440 including the oxidesemiconductor layer 403, a current value in an off state (an off-statecurrent value) can be reduced. Accordingly, an electrical signal such asan image signal can be held for a longer period in the pixel, and awriting interval can be set longer in an on state. Accordingly,frequency of refresh operation can be reduced, which leads to an effecton suppressing power consumption.

Further, in the transistors 410, 420, 430, and 440 each including theoxide semiconductor layer 403, relatively high field-effect mobility canbe obtained, whereby high-speed operation is possible. Therefore, byusing any of the transistors in a pixel portion of a liquid crystaldisplay device, a high-quality image can be provided. Since thetransistors can be separately formed over one substrate in a circuitportion and a pixel portion, the number of components can be reduced inthe liquid crystal display device.

Although there is no particular limitation on a substrate used for thesubstrate 400 having an insulating surface, a glass substrate of bariumborosilicate glass, aluminoborosilicate glass, or the like is used.

In the bottom-gate transistors 410, 420, and 430, an insulating filmserving as a base film may be provided between the substrate and thegate electrode layer. The base film has a function of preventingdiffusion of an impurity element from the substrate, and can be formedto have a single-layer structure or a stacked-layer structure using oneor more of a silicon nitride film, a silicon oxide film, a siliconnitride oxide film, and a silicon oxynitride film.

The gate electrode layer 401 can be formed to have a single-layer orstacked-layer structure using a metal material such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, neodymium, orscandium, or an alloy material which contains any of these materials asits main component.

The gate insulating layer 402 can be formed to have a single-layer or astacked-layer structure including a silicon oxide layer, a siliconnitride layer, a silicon oxynitride layer, a silicon nitride oxidelayer, an aluminum oxide layer, an aluminum nitride layer, an aluminumoxynitride layer, an aluminum nitride oxide layer, or a hafnium oxidelayer by a plasma CVD method, a sputtering method, or the like. Forexample, by a plasma CVD method, a silicon nitride layer (SiN_(y) (y>0))with a thickness of greater than or equal to 50 nm and less than orequal to 200 nm is formed as a first gate insulating layer, and asilicon oxide layer (SiO_(x) (x>0)) with a thickness of greater than orequal to 5 nm and less than or equal to 300 nm is formed as a secondgate insulating layer over the first gate insulating layer, so that agate insulating layer with a total thickness of 200 nm is formed.

A conductive film used for the source electrode layer 405 a and thedrain electrode layer 405 b can be formed using an element selected fromAl, Cr, Cu, Ta, Ti, Mo, and W, an alloy film containing any of theseelements, an alloy film containing a combination of any of theseelements, or the like. Alternatively, a structure may be employed inwhich a high-melting-point metal layer of Ti, Mo, W, or the like isstacked over and/or below a metal layer of Al, Cu, or the like. Inaddition, heat resistance can be improved by using an Al material towhich an element (Si, Nd, Sc, or the like) which prevents generation ofa hillock or a whisker in an Al film is added.

A material similar to that of the source electrode layer 405 a and thedrain electrode layer 405 b can be used for a conductive film such asthe wiring layer 436 a and the wiring layer 436 b which are connected tothe source electrode layer 405 a and the drain electrode layer 405 b,respectively.

Alternatively, the conductive film which serves as the source electrodelayer 405 a and the drain electrode layer 405 b (including a wiringformed using the same layer as the source electrode layer 405 a and thedrain electrode layer 405 b) may be formed using a conductive metaloxide. As the conductive metal oxide, indium oxide (In₂O₃), tin oxide(SnO₂), zinc oxide (ZnO), indium oxide-tin oxide alloy (In₂O₃—SnO₂,which is abbreviated to ITO), indium oxide-zinc oxide alloy (In₂O₃—ZnO),or any of these metal oxide materials in which silicon oxide iscontained can be used.

As the insulating films 407, 427, and 437, typically, an inorganicinsulating film such as a silicon oxide film, a silicon oxynitride film,an aluminum oxide film, or an aluminum oxynitride film can be used.

As the protective insulating layer 409, an inorganic insulating filmsuch as a silicon nitride film, an aluminum nitride film, a siliconnitride oxide film, or an aluminum nitride oxide film can be used.

A planarization insulating film may be formed over the protectiveinsulating layer 409 in order to reduce surface roughness due to thetransistor. As the planarization insulating film, an organic materialsuch as polyimide, acrylic, or benzocyclobutene can be used. Other thansuch organic materials, it is also possible to use a low-dielectricconstant material (a low-k material) or the like. Note that theplanarization insulating film may be formed by stacking a plurality ofinsulating films formed from these materials.

Thus, in this embodiment, by using the transistor including the oxidesemiconductor layer having a low off-state current value, a liquidcrystal display device with low power consumption can be provided.

Embodiment 4

In this embodiment, an example of a transistor including an oxidesemiconductor layer and an example of a manufacturing method thereof aredescribed in detail with reference to FIGS. 8A to 8E. The same portionsas those in the above embodiments and portions having functions similarto those of the portions in the above embodiments and steps similar tothose in the above embodiments may be handled as in the aboveembodiments, and repeated description is omitted. In addition, detaileddescription of the same portions is not repeated.

FIGS. 8A to 8E illustrate an example of a cross-sectional structure of atransistor. A transistor 510 illustrated in FIGS. 8A to 8E is abottom-gate inverted-staggered thin film transistor which is similar tothe transistor 410 illustrated in FIG. 7A.

An oxide semiconductor used for a semiconductor layer in this embodimentis an i-type (intrinsic) oxide semiconductor or a substantially i-type(intrinsic) oxide semiconductor. The i-type (intrinsic) oxidesemiconductor or substantially i-type (intrinsic) oxide semiconductor isobtained in such a manner that hydrogen, which is an n-type impurity, isremoved from an oxide semiconductor, and the oxide semiconductor ishighly purified so as to contain as few impurities that are not maincomponents of the oxide semiconductor as possible. In other words, ahighly-purified i-type (intrinsic) semiconductor or a semiconductorclose thereto is obtained not by adding impurities but by removingimpurities such as hydrogen or water as much as possible. Accordingly,the oxide semiconductor layer included in the transistor 510 is an oxidesemiconductor layer that is highly purified and made to be electricallyi-type (intrinsic).

In addition, a highly-purified oxide semiconductor includes extremelyfew carriers (close to zero), and the carrier concentration thereof isless than 1×10¹⁴/cm³, preferably less than 1×10¹²/cm³, furtherpreferably less than 1×10¹¹/cm³.

Since the oxide semiconductor includes extremely few carriers, anoff-state current can be reduced. The smaller the amount of off-statecurrent is, the better.

Specifically, in the thin film transistor including the oxidesemiconductor layer, an off-state current density per micrometer in achannel width at room temperature can be less than or equal to 10 aA/μm(1×10⁻¹⁷ A/μm), further less than or equal to 1 aA/μm (1×10⁻¹⁸ A/μm), orstill further less than or equal to 10 zA/μm (1×10⁻²⁰ A/μm).

When a transistor whose current value in an off state (an off-statecurrent value) is extremely small is used as a transistor in the pixelportion of Embodiment 1, refresh operation in a still image region canbe performed with a small number of times of writing image data.

In addition, in the transistor 510 including the above-described oxidesemiconductor layer, the temperature dependence of on-state current ishardly observed, and the off-state current remains extremely small.

Steps of manufacturing the transistor 510 over a substrate 505 aredescribed below with reference to FIGS. 8A to 8E.

First, a conductive film is formed over the substrate 505 having aninsulating surface, and then, a gate electrode layer 511 is formedthrough a first photolithography step. Note that a resist mask may beformed by an inkjet method. Formation of the resist mask by an inkjetmethod needs no photomask; thus, manufacturing cost can be reduced.

As the substrate 505 having an insulating surface, a substrate similarto the substrate 400 described in Embodiment 3 can be used. In thisembodiment, a glass substrate is used as the substrate 505.

An insulating film serving as a base film may be provided between thesubstrate 505 and the gate electrode layer 511. The base film has afunction of preventing diffusion of an impurity element from thesubstrate 505, and can be formed with a single-layer structure or astacked-layer structure using one or more of a silicon nitride film, asilicon oxide film, a silicon nitride oxide film, and a siliconoxynitride film.

In addition, the gate electrode layer 511 can be formed to have asingle-layer or stacked-layer structure using a metal material such asmolybdenum, titanium, chromium, tantalum, tungsten, aluminum, copper,neodymium, or scandium, or an alloy material which contains any of thesematerials as its main component.

Next, a gate insulating layer 507 is formed over the gate electrodelayer 511. The gate insulating layer 507 can be formed to have asingle-layer structure or a stacked-layer structure using a siliconoxide layer, a silicon nitride layer, a silicon oxynitride layer, asilicon nitride oxide layer, an aluminum oxide layer, an aluminumnitride layer, an aluminum oxynitride layer, an aluminum nitride oxidelayer, or a hafnium oxide layer, by a plasma CVD method, a sputteringmethod, or the like.

As the oxide semiconductor layer in this embodiment, an oxidesemiconductor which is made to be an i-type or substantially i-type byremoving impurities is used. Such a highly-purified oxide semiconductoris extremely sensitive to an interface level or interface charge;therefore, an interface between the oxide semiconductor layer and thegate insulating layer is important. For that reason, the gate insulatinglayer that is to be in contact with a highly-purified oxidesemiconductor needs to have high quality.

For example, a high-density plasma CVD method using microwaves (e.g., afrequency of 2.45 GHz) is preferably adopted because an insulating layercan be dense and can have high dielectric withstand voltage and highquality. When a highly-purified oxide semiconductor and a high-qualitygate insulating layer are in close contact with each other, theinterface level can be reduced and interface characteristics can befavorable.

Needless to say, another deposition method such as a sputtering methodor a plasma CVD method can be employed as long as a high-qualityinsulating layer can be formed as a gate insulating layer. Moreover, itis possible to use as the gate insulating layer an insulating layerwhose quality and characteristics of an interface with an oxidesemiconductor are improved with heat treatment performed after theformation of the insulating layer. In any case, an insulating layer thatcan reduce interface level density with an oxide semiconductor to form afavorable interface, as well as having favorable film quality as thegate insulating layer, is formed.

Further, in order that hydrogen, a hydroxyl group, and moisture might becontained in the gate insulating layer 507 and an oxide semiconductorfilm 530 as little as possible, it is preferable that the substrate 505over which the gate electrode layer 511 is formed or the substrate 505over which layers up to the gate insulating layer 507 are formed bepreheated in a preheating chamber of a sputtering apparatus aspretreatment for deposition of the oxide semiconductor film 530 so thatimpurities such as hydrogen and moisture adsorbed to the substrate 505are eliminated and evacuation is performed. As an evacuation unitprovided in the preheating chamber, a cryopump is preferable. Note thatthis preheating treatment can be omitted. This preheating step may besimilarly performed on the substrate 505 over which components up to andincluding a source electrode layer 515 a and a drain electrode layer 515b are formed before formation of an insulating layer 516.

Next, the oxide semiconductor film 530 having a thickness of greaterthan or equal to 2 nm and less than or equal to 200 nm, preferablygreater than or equal to 5 nm and less than or equal to 30 nm is formedover the gate insulating layer 507 (see FIG. 8A).

Note that before the oxide semiconductor film 530 is formed by asputtering method, powder substances (also referred to as particles ordust) which are attached on a surface of the gate insulating layer 507are preferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which voltage is applied to a substrate side, not to a targetside, using an RF power source in an argon atmosphere and plasma isgenerated in the vicinity of the substrate so that a substrate surfaceis modified. Note that instead of an argon atmosphere, a nitrogenatmosphere, a helium atmosphere, an oxygen atmosphere, or the like maybe used.

As an oxide semiconductor used for the oxide semiconductor film 530, anoxide semiconductor described in Embodiment 3, such as an oxide of fourmetal elements, an oxide of three metal elements, an oxide of two metalelements, an In—O-based oxide semiconductor, a Sn—O-based oxidesemiconductor, or a Zn—O-based oxide semiconductor can be used. Further,SiO₂ may be contained in the above oxide semiconductor. In thisembodiment, the oxide semiconductor film 530 is deposited by asputtering method with the use of an In—Ga—Zn—O-based oxidesemiconductor target. A cross-sectional view of this stage isillustrated in FIG. 8A. Alternatively, the oxide semiconductor film 530can be formed by a sputtering method in a rare gas (typically, argon)atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gasand oxygen.

As a target for manufacturing the oxide semiconductor film 530 by asputtering method, for example, a target having a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] can be used. Alternatively, a targethaving a composition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] orIn₂O₃:Ga₂O₃:ZnO=1:1:4 [molar ratio] may be used. The fill rate of theoxide target is higher than or equal to 90% and lower than or equal to100%, preferably, higher than or equal to 95% and lower than or equal to99.9%. With use of the metal oxide target with high fill rate, thedeposited oxide semiconductor film has high density.

It is preferable that a high-purity gas in which an impurity such ashydrogen, water, a hydroxyl group, or hydride is removed be used as thesputtering gas for the deposition of the oxide semiconductor film 530.

The substrate is placed in a deposition chamber under reduced pressure,and the substrate temperature is set to higher than or equal to 100° C.and lower than or equal to 600° C., preferably higher than or equal to200° C. and lower than or equal to 400° C. Deposition is performed whilethe substrate is heated, whereby the concentration of an impuritycontained in the oxide semiconductor layer formed can be reduced. Inaddition, damage by sputtering can be reduced. Then, a sputtering gasfrom which hydrogen and moisture are removed is introduced into thedeposition chamber while moisture which remains therein is removed, andthe oxide semiconductor film 530 is formed over the substrate 505 withthe use of the above target. In order to remove the moisture thatremains in the deposition chamber, an entrapment vacuum pump, forexample, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used. The evacuation unit may be a turbo molecular pumpprovided with a cold trap. In the deposition chamber which is evacuatedwith the cryopump, for example, a compound containing a hydrogen atom,water (H₂O), (further preferably, also a compound containing a carbonatom), and the like are evacuated, whereby the concentration of animpurity in the oxide semiconductor film formed in the depositionchamber can be reduced.

As one example of the deposition condition, the distance between thesubstrate and the target is 100 mm, the pressure is 0.6 Pa, thedirect-current (DC) power source is 0.5 kW, and the atmosphere is anoxygen atmosphere (the proportion of the oxygen flow rate is 100%). Notethat a pulse direct-current power source is preferable because powdersubstances (also referred to as particles or dust) generated indeposition can be reduced and the film thickness can be uniform.

Next, the oxide semiconductor film 530 is processed into anisland-shaped oxide semiconductor layer through a secondphotolithography step. A resist mask for forming the island-shaped oxidesemiconductor layer may be formed by an inkjet method. Formation of theresist mask by an inkjet method needs no photomask; thus, manufacturingcost can be reduced.

In the case where a contact hole is formed in the gate insulating layer507, a step of forming the contact hole can be performed at the sametime as processing of the oxide semiconductor film 530.

For the etching of the oxide semiconductor film 530 here, either one orboth of wet etching and dry etching may be employed. As an etchant usedfor wet etching of the oxide semiconductor film 530, for example, amixed solution of phosphoric acid, acetic acid, and nitric acid, or thelike can be used. Alternatively, ITO07N (produced by KANTO CHEMICAL CO.,INC.) may be used.

Next, first heat treatment is performed on the oxide semiconductorlayer. The oxide semiconductor layer can be dehydrated or dehydrogenatedby this first heat treatment. The temperature of the first heattreatment is higher than or equal to 400° C. and lower than or equal to750° C., or higher than or equal to 400° C. and lower than the strainpoint of the substrate. Here, the substrate is put in an electricfurnace which is a kind of heat treatment apparatus and heat treatmentis performed on the oxide semiconductor layer at 450° C. for one hour ina nitrogen atmosphere, and then, water or hydrogen is prevented fromentering the oxide semiconductor layer without exposure to the air;thus, an oxide semiconductor layer 531 is obtained (see FIG. 8B).

Note that a heat treatment apparatus is not limited to an electricalfurnace, and an apparatus for heating an object to be treated by heatconduction or heat radiation from a heating element such as a resistanceheating element may be used. For example, a rapid thermal anneal (RTA)apparatus such as a gas rapid thermal anneal (GRTA) apparatus or a lamprapid thermal anneal (LRTA) apparatus can be used. An LRTA apparatus isan apparatus for heating an object to be treated by radiation of light(an electromagnetic wave) emitted from a lamp such as a halogen lamp, ametal halide lamp, a xenon arc lamp, a carbon arc lamp, a high-pressuresodium lamp, or a high-pressure mercury lamp. A GRTA apparatus is anapparatus for heat treatment using a high-temperature gas. As thehigh-temperature gas, an inert gas that does not react with an object tobe treated by heat treatment, such as nitrogen or a rare gas like argon,is used.

For example, as the first heat treatment, GRTA in which the substrate ismoved into an inert gas heated to a high temperature higher than orequal to 650° C. and lower than or equal to 700° C., heated for severalminutes, and moved out of the inert gas heated to the high temperaturemay be performed.

Note that in the first heat treatment, it is preferable that water,hydrogen, and the like be not contained in the nitrogen or a rare gassuch as helium, neon, or argon. It is preferable that the purity ofnitrogen or the rare gas such as helium, neon, or argon which isintroduced into a heat treatment apparatus be set to be 6N (99.9999%) orhigher, preferably 7N (99.99999%) or higher (that is, the concentrationof impurities is 1 ppm or lower, preferably 0.1 ppm or lower).

Further, after the oxide semiconductor layer is heated in the first heattreatment, a high-purity oxygen gas, a high-purity N₂O gas, or anultra-dry air (the dew point is lower than or equal to −40° C.,preferably lower than or equal to −60° C.) may be introduced into thesame furnace. It is preferable that water, hydrogen, and the like be notcontained in an oxygen gas or an N₂O gas. The purity of the oxygen gasor the N₂O gas that is introduced into the heat treatment apparatus ispreferably greater than or equal to 6N, more preferably greater than orequal to 7N (i.e., the concentration of impurities in the oxygen gas orthe N₂O gas is preferably less than or equal to 1 ppm, more preferablyless than or equal to 0.1 ppm). By the action of the oxygen gas or theN₂O gas, oxygen which has been reduced at the same time as the step forremoving impurities by dehydration or dehydrogenation is supplied, sothat the oxide semiconductor layer can be a highly-purified andelectrically i-type (intrinsic) oxide semiconductor.

In addition, the first heat treatment of the oxide semiconductor layercan also be performed on the oxide semiconductor film 530 that has notyet been processed into the island-shaped oxide semiconductor layer. Inthat case, the substrate is taken out from the heat apparatus after thefirst heat treatment, and then a photolithography step is performed.

Note that the first heat treatment may be performed at any of thefollowing timings in addition to the above timing as long as it isperformed after deposition of the oxide semiconductor layer: after asource electrode layer and a drain electrode layer are formed over theoxide semiconductor layer and after an insulating layer is formed overthe source electrode layer and the drain electrode layer.

Further, the step of forming the contact hole in the gate insulatinglayer 507 may be performed either before or after the first heattreatment is performed on the semiconductor film 530.

In addition, as the oxide semiconductor layer, an oxide semiconductorlayer having a crystal region with a large thickness (a single crystalregion), that is, a crystal region which is c-axis-alignedperpendicularly to a surface of the film may be formed by performingdeposition twice and heat treatment twice, even when any of an oxide, anitride, a metal, or the like is used for a material of a basecomponent. For example, a first oxide semiconductor film with athickness greater than or equal to 3 nm and less than or equal to 15 nmis deposited, and first heat treatment is performed in a nitrogen, anoxygen, a rare gas, or a dry air atmosphere at a temperature higher thanor equal to 450° C. and lower than or equal to 850° C. or preferablyhigher than or equal to 550° C. and lower than or equal to 750° C., sothat a first oxide semiconductor film having a crystal region (includinga plate-like crystal) in a region including a surface is formed. Then, asecond oxide semiconductor film which has a larger thickness than thefirst oxide semiconductor film is formed, and second heat treatment isperformed at a temperature higher than or equal to 450° C. and lowerthan or equal to 850° C. or preferably higher than or equal to 600° C.and lower than or equal to 700° C., so that crystal growth proceedsupward with the use of the first oxide semiconductor film as a seed ofthe crystal growth and the whole second oxide semiconductor film iscrystallized. In such a manner, the oxide semiconductor layer having acrystal region having a large thickness may be formed.

Next, a conductive film serving as the source electrode layer 515 a andthe drain electrode layer 515 b (including a wiring formed in the samelayer as the source electrode layer 515 a and the drain electrode layer515 b) is formed over the gate insulating layer 507 and the oxidesemiconductor layer 531. As the conductive film serving as the sourceelectrode layer 515 a and the drain electrode layer 515 b, the materialused for the source electrode layer 405 a and the drain electrode layer405 b which is described in Embodiment 3 can be used.

A resist mask is formed over the conductive film through a thirdphotolithography step, and the source electrode layer 515 a and thedrain electrode layer 515 b are formed by selective etching, and then,the resist mask is removed (see FIG. 8C).

Light exposure at the time of the formation of the resist mask in thethird photolithography step may be performed using ultraviolet light,KrF laser light, or ArF laser light. A channel length L of a transistorthat is completed later is determined by a distance between bottom endportions of the source electrode layer and the drain electrode layer,which are adjacent to each other over the oxide semiconductor layer 531.In the case where light exposure is performed for a channel length L ofless than 25 nm, the light exposure at the time of the formation of theresist mask in the third photolithography step may be performed usingextreme ultraviolet having an extremely short wavelength of severalnanometers to several tens of nanometers. Light exposure with extremeultraviolet leads to a high resolution and a large depth of focus. Thus,the channel length L of the transistor that is completed later can begreater than or equal to 10 nm and less than or equal to 1000 nm and theoperation speed of a circuit can be increased.

In order to reduce the number of photomasks used in a photolithographystep and reduce the number of photolithography steps, an etching stepmay be performed with the use of a multi-tone mask that is alight-exposure mask through which light is transmitted to have variousintensities. A resist mask formed with the use of a multi-tone mask hasvarious thicknesses and further can be changed in shape by etching;therefore, the resist mask can be used in a plurality of etching stepsfor processing into different patterns. Therefore, a resist maskcorresponding to at least two kinds or more of different patterns can beformed by one multi-tone mask. Thus, the number of light-exposure maskscan be reduced and the number of corresponding photolithography stepscan be also reduced, whereby simplification of a process can berealized.

Note that it is preferable that etching conditions be optimized so asnot to etch and divide the oxide semiconductor layer 531 when theconductive film is etched. However, it is difficult to obtain etchingconditions in which only the conductive film is etched and the oxidesemiconductor layer 531 is not etched at all. In some cases, only partof the oxide semiconductor layer 531 is etched when the conductive filmis etched, whereby the oxide semiconductor layer 531 having a grooveportion (a recessed portion) is formed.

In this embodiment, since the Ti film is used as the conductive film andthe In—Ga—Zn—O-based oxide semiconductor is used as the oxidesemiconductor layer 531, ammonia hydrogen peroxide (a mixed solution ofammonia, water, and hydrogen peroxide) is used as an etchant for etchingthe conductive film.

Next, by plasma treatment using a gas such as N₂O, N₂, or Ar, water orthe like adsorbed to a surface of an exposed portion of the oxidesemiconductor layer may be removed. In the case where the plasmatreatment is performed, the insulating layer 516 is formed withoutexposure to the air as a protective insulating film in contact with partof the oxide semiconductor layer.

The insulating layer 516 can be formed to a thickness of at least 1 nmby a method by which an impurity such as water or hydrogen does notenter the insulating layer 516, such as a sputtering method asappropriate. When hydrogen is contained in the insulating layer 516,entry of the hydrogen to the oxide semiconductor layer, or extraction ofoxygen in the oxide semiconductor layer by the hydrogen may occur,thereby causing the backchannel of the oxide semiconductor layer to havelower resistance (to be n-type), so that a parasitic channel may beformed. Therefore, it is important that a deposition method in whichhydrogen is not used is employed in order to form the insulating layer516 containing as little hydrogen as possible.

In this embodiment, a silicon oxide film is formed to a thickness of 200nm as the insulating layer 516 with a sputtering method. The substratetemperature in deposition may be higher than or equal to roomtemperature and lower than or equal to 300° C. and in this embodiment,is 100° C. The silicon oxide film can be deposited by a sputteringmethod in a rare gas (typically, argon) atmosphere, an oxygenatmosphere, or a mixed atmosphere containing a rare gas and oxygen. As atarget, a silicon oxide target or a silicon target may be used. Forexample, the silicon oxide film can be formed using a silicon target bya sputtering method in an atmosphere containing oxygen. As theinsulating layer 516 that is formed in contact with the oxidesemiconductor layer, an inorganic insulating film which does not includeimpurities such as moisture, a hydrogen ion, and OH⁻ and blocks entry ofthese from the outside is used. Typically, a silicon oxide film, asilicon oxynitride film, an aluminum oxide film, an aluminum oxynitridefilm, or the like is used.

In order to remove residual moisture in the deposition chamber of theinsulating layer 516 at the same time as deposition of the oxidesemiconductor film 530, an entrapment vacuum pump (such as a cryopump)is preferably used. When the insulating layer 516 is deposited in thedeposition chamber evacuated using a cryopump, the impurityconcentration in the insulating layer 516 can be reduced. In addition,as an evacuation unit for removing the residual moisture in thedeposition chamber of the insulating layer 516, a turbo molecular pumpprovided with a cold trap may be used.

It is preferable that a high-purity gas in which an impurity such ashydrogen, water, a hydroxyl group, or hydride is removed be used as thesputtering gas for the deposition of the insulating layer 516.

Next, second heat treatment is performed in an inert gas atmosphere oroxygen gas atmosphere (preferably at a temperature higher than or equalto 200 and lower than or equal to 400° C., for example, higher than orequal to 250 and lower than or equal to 350° C.). For example, thesecond heat treatment is performed in a nitrogen atmosphere at 250° C.for one hour. In the second heat treatment, part of the oxidesemiconductor layer (a channel formation region) is heated while beingin contact with the insulating layer 516.

Through the above process, the first heat treatment is performed on theoxide semiconductor film so that an impurity such as hydrogen, moisture,a hydroxyl group, or hydride (also referred to as a hydrogen compound)is intentionally removed from the oxide semiconductor layer.Additionally, oxygen that is one of main components of an oxidesemiconductor and is simultaneously reduced in a step of removing animpurity can be supplied. Accordingly, the oxide semiconductor layer ishighly purified to be an electrically i-type (intrinsic) semiconductor.

Through the above process, the transistor 510 is formed (FIG. 8D).

When a silicon oxide layer having a lot of defects is used as theinsulating layer 516, heat treatment after formation of the siliconoxide layer has an effect in diffusing an impurity such as hydrogen,moisture, a hydroxyl group, or hydride contained in the oxidesemiconductor layer to the oxide insulating layer so that the impuritycontained in the oxide semiconductor layer can be further reduced.

A protective insulating layer 506 may be formed over the insulatinglayer 516. For example, a silicon nitride film is formed by an RFsputtering method. Since an RF sputtering method has high productivity,it is preferably used as a deposition method of the protectiveinsulating layer. As the protective insulating layer, an inorganicinsulating film that does not include an impurity such as moisture andprevents entry of these from the outside, such as a silicon nitride filmor an aluminum nitride film is used. In this embodiment, the protectiveinsulating layer 506 is formed using a silicon nitride film (see FIG.8E).

In this embodiment, as the protective insulating layer 506, a siliconnitride film is formed by heating the substrate 505 over which layers upto the insulating layer 516 are formed, to a temperature higher than orequal to 100° C. and lower than or equal to 400° C., introducing asputtering gas containing high-purity nitrogen from which hydrogen andmoisture are removed, and using a target of silicon semiconductor. Inthis case, the protective insulating layer 506 is preferably depositedremoving moisture remaining in a treatment chamber, similarly to theinsulating layer 516.

After the formation of the protective insulating layer 506, heattreatment may be further performed at a temperature of a temperaturehigher than or equal to 100° C. and lower than or equal to 200° C. inthe air for longer than or equal to 1 hour and shorter than or equal to30 hours. This heat treatment may be performed at a fixed heatingtemperature. Alternatively, the following change in the heatingtemperature may be conducted plural times repeatedly: the heatingtemperature is increased from a room temperature to a temperature higherthan or equal to 100° C. and lower than or equal to 200° C. and thendecreased to a room temperature.

In this manner, with the use of the transistor including ahighly-purified oxide semiconductor layer manufactured using thisembodiment, the current value in the off-state (an off-state currentvalue) can be further reduced. Accordingly, an electric signal such asimage data can be held for a longer period and a writing interval can beset longer. Therefore, the frequency of refresh operation can bereduced, which leads to a higher effect of suppressing powerconsumption.

In addition, since the transistor described in this embodiment has highfield-effect mobility, high-speed operation is possible. Accordingly, byusing the transistor in a pixel portion of a liquid crystal displaydevice, a high-quality image can be provided. In addition, since thetransistor can be separately formed in a driver circuit and a pixelportion over one substrate, the number of components of the liquidcrystal display device can be reduced.

This embodiment can be implemented combining with any of the otherembodiments as appropriate.

Embodiment 5

A liquid crystal display device disclosed in this specification can beapplied to a variety of electronic devices (including game machines).Examples of electronic devices are a television set (also referred to asa television or a television receiver), a monitor of a computer or thelike, a camera such as a digital camera or a digital video camera, adigital photo frame, a mobile phone handset (also referred to as amobile phone or a mobile phone device), a portable game machine, aportable information terminal, an audio reproducing device, alarge-sized game machine such as a pachinko machine, and the like. Inthis embodiment, examples of electronic devices including any of theliquid crystal display devices of the embodiments described above willbe described.

FIG. 9A illustrates an electronic book reader (also referred to as ane-book reader) which can include housings 9630, a display portion 9631,operation keys 9632, a solar battery 9633, and a charge and dischargecontrol circuit 9634. The electronic book reader illustrated in FIG. 9Acan have various functions such as a function of displaying variouskinds of information (e.g., a still image, a moving image, and a textimage); a function of displaying a calendar, a date, a time, and thelike on the display portion; a function of operating or editing theinformation displayed on the display portion; and a function ofcontrolling processing by various kinds of software (programs). Notethat in FIG. 9A, a structure including a battery 9635 and a DCDCconverter 9636 (hereinafter abbreviated as a converter 9636) isillustrated as an example of the charge and discharge control circuit9634. By applying the liquid crystal display device described in any ofEmbodiments 1 to 4 to the display portion 9631, an electronic bookreader which is capable of maintaining favorable display images forlonger time, with high security and low power consumption can beprovided.

In the case where a semi-transmissive liquid crystal display device or areflective liquid crystal display device be used as the display portion9631, use under a relatively bright condition is assumed; therefore, thestructure illustrated in FIG. 9A is preferable because power generationby the solar battery 9633 and charge in the battery 9635 are effectivelyperformed. Note that the solar battery 9633 can be provided on a spaceof the housing 9630 (a surface and a rear surface) as appropriate;therefore charge of the battery 9635 is efficiently performed. When alithium ion battery is used as the battery 9635, there is an advantageof downsizing or the like.

The structure and the operation of the charge and discharge controlcircuit 9634 illustrated in FIG. 9A will be described with reference toa block diagram in FIG. 9B. The solar battery 9633, the battery 9635,the converter 9636, a converter 9637, switches SW1 to SW3, and thedisplay portion 9631 are illustrated in FIG. 9B, and the battery 9635,the converter 9636, the converter 9637, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634.

First, an example of operation in the case where power is generated bythe solar battery 9633 utilizing external light is described. Thevoltage of electric power generated by the solar battery is raised orlowered by the converter 9636 to be a voltage for charging the battery9635. Then, when the electric power from the solar battery 9633 is usedfor the operation of the display portion 9631, the switch SW1 is turnedon and the voltage of the electric power is raised or lowered by theconverter 9637 so as to be a voltage needed for the display portion9631. In addition, when display on the display portion 9631 is notperformed, the switch SW1 is turned off and the switch SW2 is turned onso that charge of the battery 9635 may be performed.

Next, operation in the case where power is not generated by the solarbattery 9633 utilizing external light is described. The voltage ofelectric power accumulated in the battery 9635 is raised or lowered bythe converter 9637 by turning on the switch SW3. Then, electric powerfrom the battery 9635 is used for the operation of the display portion9631.

Note that although the solar battery 9633 is described as an example ofmeans for charge, charge of the battery 9635 may be performed withanother means. In addition, a combination of the solar battery 9633 andanother means for charge may be used.

FIG. 10A illustrates a laptop personal computer, which includes a mainbody 3001, a housing 3002, a display portion 3003, a keyboard 3004, andthe like. By applying the liquid crystal display device described in anyof Embodiments 1 to 4 to the display portion 3003, a laptop personalcomputer capable of maintaining favorable display images for longertime, with high security and low power consumption can be provided.

FIG. 10B is a portable information terminal (PDA) which includes adisplay portion 3023, an external interface 3025, an operation button3024, and the like in a main body 3021. In addition, a stylus 3022 isincluded as an accessory for operation. By applying the liquid crystaldisplay device described in any of Embodiments 1 to 4 to the displayportion 3023, a portable information terminal (PDA) in which convenienceand security is improved and power consumption is reduced can beprovided.

FIG. 10C illustrates an example of an electronic book reader. Forexample, the electronic book reader 2700 includes two housings, ahousing 2701 and a housing 2703. The housing 2701 and the housing 2703are combined with a hinge 2711 so that the electronic book reader 2700can be opened and closed with the hinge 2711 as an axis. With such astructure, the electronic book reader 2700 can operate like a paperbook.

A display portion 2705 and a display portion 2707 are incorporated inthe housing 2701 and the housing 2703, respectively. The display portion2705 and the display portion 2707 may display one image or differentimages. In the case where the display portion 2705 and the displayportion 2707 display different images, for example, text can bedisplayed on a display portion on the right side (the display portion2705 in FIG. 10C) and graphics can be displayed on a display portion onthe left side (the display portion 2707 in FIG. 10C). By applying theliquid crystal display device described in any of Embodiments 1 to 4 tothe display portion 2705 and the display portion 2707, a laptop personalcomputer capable of maintaining favorable display images for longertime, with high security and low power consumption can be provided.

FIG. 10C illustrates an example in which the housing 2701 is providedwith an operation portion and the like. For example, the housing 2701 isprovided with a power switch 2721, operation keys 2723, a speaker 2725,and the like. With the operation key 2723, pages can be turned. Notethat a keyboard, a pointing device, or the like may also be provided onthe surface of the housing, on which the display portion is provided.Furthermore, an external connection terminal (an earphone terminal, aUSB terminal, or the like), a recording medium insertion portion, andthe like may be provided on the back surface or the side surface of thehousing. Moreover, the electronic book reader 2700 may have a functionof an electronic dictionary.

The electronic book reader 2700 may have a configuration capable ofwirelessly transmitting and receiving data. Through wirelesscommunication, desired book data or the like can be purchased anddownloaded from an electronic book server.

FIG. 10D illustrates a mobile phone, which includes two housings, ahousing 2800 and a housing 2801. The housing 2801 includes a displaypanel 2802, a speaker 2803, a microphone 2804, a pointing device 2806, acamera lens 2807, an external connection terminal 2808, and the like. Inaddition, the housing 2800 includes a solar cell 2810 having a functionof charge of the portable information terminal, an external memory slot2811, and the like. Further, an antenna is incorporated in the housing2801. By applying the liquid crystal display device described in any ofEmbodiments 1 to 4 to the display panel 2802, a mobile phone capable ofmaintaining favorable display images for longer time, with high securityand low power consumption can be provided.

Further, the display panel 2802 is provided with a touch panel. Aplurality of operation keys 2805 which are displayed as images isillustrated by dashed lines in FIG. 10D. Note that a boosting circuit bywhich a voltage output from the solar cell 2810 is increased to besufficiently high for each circuit is also included.

In the display panel 2802, the display direction can be appropriatelychanged depending on a usage pattern. Further, the display device isprovided with the camera lens 2807 on the same surface as the displaypanel 2802, and thus it can be used as a video phone. The speaker 2803and the microphone 2804 can be used for videophone calls, recording andplaying sound, and the like as well as voice calls. Moreover, thehousings 2800 and 2801 in a state where they are opened as illustratedin FIG. 10D can be slid so that one is lapped over the other; therefore,the size of the mobile phone can be reduced, which makes the mobilephone suitable for being carried.

The external connection terminal 2808 can be connected to an AC adapterand various types of cables such as a USB cable, and charging and datacommunication with a personal computer are possible. Moreover, a largeamount of data can be stored by inserting a storage medium into theexternal memory slot 2811 and can be moved.

Further, in addition to the above functions, an infrared communicationfunction, a television reception function, or the like may be provided.

FIG. 10E illustrates a digital video camera which includes a main body3051, a display portion A 3057, an eyepiece 3053, an operation switch3054, a display portion B 3055, a battery 3056, and the like. Byapplying the liquid crystal display device described in any ofEmbodiments 1 to 4 to the display portion A 3057 and the display portionB 3055, a digital video camera capable of maintaining favorable displayimages for longer time, with high security and low power consumption canbe provided.

FIG. 10F illustrates an example of a television set. In the televisionset 9600, a display portion 9603 is incorporated in a housing 9601. Thedisplay portion 9603 can display images. Here, the housing 9601 issupported by a stand 9605. By applying the liquid crystal display devicedescribed in any of Embodiments 1 to 4 to the display portion 9603, thetelevision set capable of maintaining favorable display images forlonger time, with high security and low power consumption can provided.

The television set 9600 can be operated by an operation switch of thehousing 9601 or a separate remote controller. Further, the remotecontroller may be provided with a display portion for displaying dataoutput from the remote controller.

Note that the television set 9600 is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the display device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

This embodiment can be implemented in appropriate combination with thestructures described in the other embodiments.

Example 1

In this example, results obtained by comparing display states betweenfollowing liquid crystal display devices are described: a liquid crystaldisplay device in which an initial-state image is displayed before theliquid crystal display device is turned off: and a liquid crystaldisplay device in which an image before the liquid crystal displaydevice is in an off state is being displayed when the liquid crystaldisplay device is turned off as a comparative example.

FIG. 12A and FIG. 13A (FIG. 12A and FIG. 13A are the same pictures) arepictures of a display screen on which an on-state image before theliquid crystal display device is in the off state is displayed. Imagesin FIG. 12A and FIG. 13A are checkered pattern of black and white. As aswitching element of a pixel, a transistor including an oxidesemiconductor layer (In—Ga—Zn—O layer) having low off-state current wasemployed. The liquid crystal display device of this example is atransmissive liquid crystal display device, in which light is suppliedfrom a backlight. In this example, after the liquid crystal displaydevice was turned off to stop the supply of a power source potential toa display panel including a driver circuit portion and a pixel portion,the backlight remains on so that the display state of the screen couldbe recognized. Since the liquid crystal display device of this exampleis a normally-white liquid crystal display device, a display in whichliquid crystals are in an initial state is white by transmitting lightfrom the backlight.

In FIG. 12B, a display screen just after the display device is turnedoff in the case where a fixed potential is written to the capacitor toreturn the liquid crystal to the initial state before the display deviceis turned off, and then the supply of the power source potential to thedisplay panel including the driver circuit portion and the pixel portionis stopped is illustrated. On the display screen, an all-whiteinitial-state image displayed when the liquid crystals are in theinitial state is displayed. Therefore, it is found that, in the offstate, the liquid crystals are in a stable initial state where electricfield is not applied.

On the other hand, FIG. 13B illustrates a display screen just after theliquid crystal display device is turned off, in the case where theliquid crystal display device is turned off to stop the supply of thepower source potential to the display panel while keeping the displayimage of the checked pattern illustrated in FIG. 13A left, as acomparative example. In FIG. 13B, it is found that the image of thecheckered pattern displayed in the on state just before the liquidcrystal display device is turned off can be faintly observed andelectric field is continuously applied to the liquid crystals after theliquid crystal display device is turned off. Such applying of electricfield for a period of time during which applying of electric field tothe liquid crystals is not needed causes deterioration of the liquidcrystals and the image display function and reliability of a liquidcrystal display device may be deteriorated.

As apparent from the above, before the liquid crystal display device isin the off state, the fixed potential is written such that voltage isnot applied to the liquid crystal element, and an initial-state image isdisplayed; therefore the liquid crystal element can be prevented fromdeteriorating, favorable image display function can be maintained forlong period, and security can be improved.

Accordingly, a highly reliable liquid crystal display device in whichlow power consumption is achieved and a driving method of the liquidcrystal display device can be provided.

This application is based on Japanese Patent Application serial No.2010-009853 filed with Japan Patent Office on Jan. 20, 2010, the entirecontents of which are hereby incorporated by reference.

1. A driving method of a liquid crystal display device, comprising thesteps of: displaying an image on a screen provided with pixels eachincluding a capacitor, a liquid crystal element, and a semiconductorelement by supplying a power source potential from a power source sothat liquid crystals of the liquid crystal element responds; supplying astop signal by a stop unit; writing a fixed potential to the capacitorin each of the pixels in accordance with the stop signal so that a stateof the liquid crystals changes from a response state to a non-responsestate; and stopping the supply of the power source potential from thepower source after writing the fixed potential to the capacitor, whereinthe semiconductor element comprises an oxide semiconductor.
 2. Thedriving method of a liquid crystal display device according to claim 1,wherein an initial-state image is displayed on the screen by writing thefixed potential to the capacitor in each of the pixels.
 3. The drivingmethod of a liquid crystal display device according to claim 2, whereinan all white image is displayed on the screen as the initial-stateimage.
 4. The driving method of a liquid crystal display deviceaccording to claim 2, wherein an all black image is displayed on thescreen as the initial-state image.
 5. The driving method of a liquidcrystal display device according to claim 1, further comprising the stepof comparing image signals in successive frame periods with respect toeach of the pixels.
 6. The driving method of a liquid crystal displaydevice according to claim 1, wherein a second image signal is notwritten in a case where a first image signal corresponding to a firstframe and the second image signal corresponding to a second frame arethe same wherein the first frame and the second frame are successiveframes.
 7. The driving method of a liquid crystal display deviceaccording to claim 1, wherein the power source potential is supplied toa driver circuit portion in the step of supplying the power sourcepotential from the power source, and wherein the supply of the powersource potential to the driver circuit portion is stopped in the step ofstopping the supply of the power source potential from the power source.8. The driving method of a liquid crystal display device according toclaim 1, wherein the power source potential is supplied to a drivercircuit portion and a backlight portion in the step of supplying thepower source potential from the power source, and wherein the supply ofthe power source potential to the driver circuit portion and thebacklight portion is stopped in the step of stopping the supply of thepower source potential from the power source.
 9. A driving method of aliquid crystal display device, comprising the steps of: displaying animage on a screen provided with pixels each including a capacitor, aliquid crystal element, and a semiconductor element by supplying a powersource potential from a power source so that liquid crystals of theliquid crystal element responds; supplying a stop signal by a stop unit;writing a fixed potential to the capacitor in each of the pixels inaccordance with the stop signal so that a state of the liquid crystalschanges from a response state to a non-response state; and stopping thesupply of the power source potential from the power source after writingthe fixed potential to the capacitor.
 10. The driving method of a liquidcrystal display device according to claim 9, wherein a transistorincluding an oxide semiconductor layer is used as the semiconductorelement, and wherein the capacitor and the liquid crystal element areelectrically connected in parallel.
 11. The driving method of a liquidcrystal display device according to claim 9, wherein an initial-stateimage is displayed on the screen by writing the fixed potential to thecapacitor in each of the pixels.
 12. The driving method of a liquidcrystal display device according to claim 11, wherein an all white imageis displayed on the screen as the initial-state image.
 13. The drivingmethod of a liquid crystal display device according to claim 11, whereinan all black image is displayed on the screen as the initial-stateimage.
 14. The driving method of a liquid crystal display deviceaccording to claim 9, further comprising the step of comparing imagesignals in successive frame periods with respect to each of the pixels.15. The driving method of a liquid crystal display device according toclaim 9, wherein a second image signal is not written in a case where afirst image signal corresponding to a first frame and the second imagesignal corresponding to a second frame are the same wherein the firstframe and the second frame are successive frames.
 16. The driving methodof a liquid crystal display device according to claim 9, wherein thepower source potential is supplied to a driver circuit portion in thestep of supplying the power source potential from the power source, andwherein the supply of the power source potential to the driver circuitportion is stopped in the step of stopping the supply of the powersource potential from the power source.
 17. The driving method of aliquid crystal display device according to claim 9, wherein the powersource potential is supplied to a driver circuit portion and a backlightportion in the step of supplying the power source potential from thepower source, and wherein the supply of the power source potential tothe driver circuit portion and the backlight portion is stopped in thestep of stopping the supply of the power source potential from the powersource.
 18. A driving method of a liquid crystal display device,comprising the steps of: displaying an image on a screen provided withpixels each including a capacitor, a liquid crystal element, and asemiconductor element by supplying a power source potential from a powersource so that liquid crystals of the liquid crystal element responds;supplying a stop signal by a stop unit; writing a fixed potential to thecapacitor in each of the pixels in accordance with the stop signal sothat a state of the liquid crystals changes from a response state to anon-response state; and stopping the supply of the power sourcepotential from the power source after writing the fixed potential to thecapacitor, wherein the liquid crystal element comprises a nematic liquidcrystal.
 19. The driving method of a liquid crystal display deviceaccording to claim 18, wherein the semiconductor element comprises anoxide semiconductor layer.
 20. The driving method of a liquid crystaldisplay device according to claim 18, wherein a transistor including anoxide semiconductor layer is used as the semiconductor element.
 21. Thedriving method of a liquid crystal display device according to claim 18,wherein an initial-state image is displayed on the screen by writing thefixed potential to the capacitor in each of the pixels.
 22. The drivingmethod of a liquid crystal display device according to claim 21, whereinan all white image is displayed on the screen as the initial-stateimage.
 23. The driving method of a liquid crystal display deviceaccording to claim 21, wherein an all black image is displayed on thescreen as the initial-state image.
 24. The driving method of a liquidcrystal display device according to claim 18, further comprising thestep of comparing image signals in successive frame periods with respectto each of the pixels.
 25. The driving method of a liquid crystaldisplay device according to claim 18, wherein a second image signal isnot written in a case where a first image signal corresponding to afirst frame and the second image signal corresponding to a second frameare the same wherein the first frame and the second frame are successiveframes.
 26. The driving method of a liquid crystal display deviceaccording to claim 18, wherein the power source potential is supplied toa driver circuit portion in the step of supplying the power sourcepotential from the power source, and wherein the supply of the powersource potential to the driver circuit portion is stopped in the step ofstopping the supply of the power source potential from the power source.27. The driving method of a liquid crystal display device according toclaim 18, wherein the power source potential is supplied to a drivercircuit portion and a backlight portion in the step of supplying thepower source potential from the power source, and wherein the supply ofthe power source potential to the driver circuit portion and thebacklight portion is stopped in the step of stopping the supply of thepower source potential from the power source.
 28. A driving method of aliquid crystal display device, comprising the steps of: displaying animage on a screen provided with pixels each including a capacitor, aliquid crystal element, and a semiconductor element by supplying a powersource potential from a power source to a driver circuit portion and abacklight portion so that liquid crystals of the liquid crystal elementresponds; supplying a stop signal by a stop unit; stopping the supply ofthe power source potential from the power source to the backlightportion; writing a fixed potential to the capacitor in each of thepixels in accordance with the stop signal so that a state of the liquidcrystals changes from a response state to a non-response state; andstopping the supply of the power source potential from the power sourceto the driver circuit portion after wiring the fixed potential to thecapacitor.
 29. The driving method of a liquid crystal display deviceaccording to claim 28, wherein the semiconductor element comprises anoxide semiconductor layer.
 30. The driving method of a liquid crystaldisplay device according to claim 28, wherein a transistor including anoxide semiconductor layer is used as the semiconductor element.
 31. Thedriving method of a liquid crystal display device according to claim 28,wherein an initial-state image is displayed on the screen by writing thefixed potential to the capacitor in each of the pixels.
 32. The drivingmethod of a liquid crystal display device according to claim 31, whereinan all white image is displayed on the screen as the initial-stateimage.
 33. The driving method of a liquid crystal display deviceaccording to claim 31, wherein an all black image is displayed on thescreen as the initial-state image
 34. The driving method of a liquidcrystal display device according to claim 28, further comprising thestep of comparing image signals in successive frame periods with respectto each of the pixels.
 35. The driving method of a liquid crystaldisplay device according to claim 28, wherein a second image signal isnot written in a case where a first image signal corresponding to afirst frame and the second image signal corresponding to a second frameare the same wherein the first frame and the second frame are successiveframes.